The present invention relates to a second harmonic generating method and apparatus, and more particularly, to a second harmonic generating method and apparatus capable of reducing a warm-up time and generating a stable second harmonic even with the variation of an ambient temperature environment.
A second harmonic generator, which adopts a frequency doubling principle within a resonator can be made compact and generate short wave visible light. Accordingly, the second harmonic generator is widely used as a light source for use in digital video recording/reproducing apparatuses, high resolution image processors, high speed data processors, etc.
Generally a second harmonic generator generates a blue-green laser. Such a second harmonic generator has a structure in which a gain medium such as Nd:YAG, a polarizer such as a Brewster plate and a non-linear birefringent device such as a KTP (KTiOPO.sub.4) are provided on a single optical axis in an intracavity which is optically confined by two mirrors opposing each other.
The gain medium generates a fundamental wave using pumping laser which is applied thereto from an external source. Then, the non-linear birefringent crystalline device generates a second harmonic using the fundamental wave. Here, the polarizer allows specifically polarized light of the fundamental wave to pass therethrough and to be incident to the non-linear birefringent crystalline device.
A diode-laser-pumped intracavity second harmonic generator generates considerable heat during its operation. In particular, the second harmonic generation characteristic of the non-linear birefringent crystalline device is variable sensitively according to such heat variation.
There are two general methods for thermally stabilizing the second harmonic generator. One is an output compensation method and the other is a mode selection method.
First, in the compensation method, the variation of the second harmonic output is compensated to match a prescribed value by regulating an output of the laser diode when such an output is deviated from a reference value while monitoring the second harmonic output. A conventional second harmonic generator having such an output compensation mechanism has such a resonator as shown in FIG. 1.
Referring to FIG. 1, an input mirror 11 having a high transmittivity with respect to the pumping laser and a high reflectivity with respect to a fundamental wave and an output mirror 15 having a high transmittivity only with respect to the second harmonic are provided at both ends of a casing 10 having an intracavity, that is a resonant section, respectively. A gain medium 12 for generating a fundamental wave using the pumping laser, a polarizer 13 for passing specifically polarized light of the fundamental wave and a non-linear birefringent crystalline device 14 for generating a second harmonic using the incident fundamental wave are provided in the resonant section. A laser diode 21 for generating the pumping laser and a focus lens 22 for focusing the pumping laser into the intracavity are provided in front of a resonator, that is, before input mirror 11. On the other hand, a beam splitter 23 is provided at the back of the resonator, that is, after output mirror 15, to thereby separately reflecting part of the harmonic into a different path. Also, a photo detector 24 for detecting the intensity of the input second harmonic is provided on the proceeding path of the second harmonic which is reflected from beam splitter 23. Photo detector 24 is electrically connected to a laser diode control circuit 20 for controlling the output of laser diode 21, and applies a monitor output of the second harmonic for controlling laser diode 21 as an electrical signal to laser diode control circuit 20. Control circuit 20 lowers the output of laser diode 21 when the second harmonic output exceeds a reference value, and increases the laser diode output when the second harmonic output is below the reference value.
As for the mode selection method, the output of the laser diode is held constant and the temperature of the non-linear birefringent crystalline device which is located in the intracavity is precisely regulated, to maintain a stable second harmonic output. A conventional second harmonic generator, having such a mode selection mechanism, is shown in FIG. 2.
Referring to FIG. 2, as described above, an input mirror 11a having a high transmittivity with respect to the pumping laser and a high reflectivity with respect to a fundamental wave and an output mirror 15a having a high transmittivity only with respect to the second harmonic are provided at both ends of a casing 10a maintaining the resonator structure, respectively. A gain medium 12a, a polarizer 13a and a non-linear birefringent crystalline device 14a are provided in a resonant section between input mirror 11a and output mirror 15a. A thermoelectric cooler 16a is provided in the lower portion of non-linear birefringent crystalline device 14a. As described above with respect to the aforementioned second harmonic generator, the intensity of the second harmonic is electrically converted and fed back to a control circuit (not shown), to regulate the temperature of non-linear birefringent crystalline device 14a.
The above actual structure is specifically disclosed in U.S. Pat. No. 3,858,056, in which part of the output light is reflected via a beam splitter and the reflected light is fed back as an electrical signal. Here, temperature of a nonlinear birefringent crystalline device is regulated by a stepping motor and a potentiometer operated by the stepping motor. However, such a structure requires a long warm-up time (more than five minutes) for preparing the second harmonic generation, and has another drawback in that precise temperature control of the nonlinear birefringent crystalline device is difficult.
Generally, to stabilize the second harmonic output to within .+-.3% of a prescribed value in the above-described mode selection method, the temperature deviation of the non-linear birefringent crystalline device should be limited to within about .+-.0.01.degree. C.
In the diode-laser-pumped intracavity second harmonic generator, a thermal stability of the material of the metal support member which maintains the resonator is a very important factor in output stability. Generally, when the thermal stability of the metal support member is lowered, the axial mode of the laser output becomes erratic, to thereby make the laser output unstable, which is called "hopping."
Therefore, it is desirable that a material having a low coefficient of linear expansion is used as that of the resonator support member. A metallic material such as aluminum whose coefficient of linear expansion is 24.times.10.sup.-6 /.degree.C. or brass whose coefficient of linear expansion is 19.times.10.sup.-6 /.degree.C. is often used as the resonator support member. Also, a glass material may be used as the resonator support member considering the cost of the material (see U.S. Pat. No. 5,170,409). However, since resonator is at most tens of millimeters in length, even though the temperature deviation is less than 1.degree.-2.degree. C. in the case of limited compactness, the output hopping phenomenon still occurs.
Thus, when the length of the resonator is extremely short, a material for a support member having an extremely low coefficient of linear expansion (below an order of 10.sup.-7 .degree.C.) should be used for preventing such a mode hopping phenomenon. Otherwise, a structure for generating an extremely small linear expansion should be provided. However, there are few materials having such a coefficient of linear expansion among those currently available materials, which is disadvantageous in view of costs.